This invention relates to positive crankcase ventilation for automotive vehicle engines and, more particularly, to a combination flow control orifice and muffler for reducing noise emitted from the PCV orifice and system.
Positive crankcase ventilation (PCV) systems have been used for many years to reduce emission of contaminants by recirculating blowby gases and crankcase vapors into the engine cylinders for burning. This is commonly accomplished by drawing the crankcase vapors through a liquid separator and conducting the resulting vapors and contaminants into the air intake or the intake manifold where it is drawn into the cylinders by intake system or intake manifold vacuum. If the crankcase vapors are drawn into an intake manifold, a PCV valve is generally required to control the amount of PCV flow because the manifold vacuum varies greatly and generally inversely to the amount of crankcase vapors which it is necessary to have recirculated. However, where the vapors are recirculated into the air intake system, it has been generally satisfactory to control the crankcase vapor flow by means of an orifice.
In the development of a new engine design, it was desired to simplify the PCV system and replace the usual PCV valve by utilizing a single orifice at the outlet from the cam cover, which encloses a volume connecting with the engine crankcase, and a fitting at an inlet to the intake manifold, the orifice and the fitting being connected by a fluid-conducting hose. It was found, however, that when the engine was operated at idle, a xe2x80x9cvacuum leakxe2x80x9d like hiss noise was radiated from the orifice. The noise level of the orifice and PCV system exceeded the background noise level of the engine in the idle condition, where intake manifold vacuum is high, and was deemed unacceptable. A modified noise-reducing PCV orifice was accordingly desired.
The present invention provides a noise-reducing positive crankcase ventilation (PCV) orifice, called hereinafter an orifice muffler. As developed, the orifice muffler substantially reduced the objectionable noise levels in the frequency range above 2500 Hz, which were responsible for the objectionable noise transmissions in the original design.
The orifice muffler of the invention comprises an elongated tube with imperforate side walls and inlet and outlet end walls. The outlet end wall is provided with a single orifice while the inlet end wall includes at least two orifices of differing sizes. The dual dissimilar inlet orifices are operative to pass the controlled gas flow and to create destructive wave interference between the flow streams through the muffler to minimize the noise caused by turbulent air flow through the muffler.
Preferably, the tube is made of steel, or a material having heat transfer characteristics near that of steel, so that the tube heats up quickly during operation and freezing of moisture in the orifices is thereby avoided. The wall thickness of the steel tube is made sufficiently thick to minimize radiated noise from the muffler. Preferably, the length of the tube ranges between about 30 and 40 millimeters to attenuate relatively high frequency noise, and the expansion ratio of the inlet orifice areas to the cross-sectional area of the tube is at least 10 to 1 to maximize attenuation of the turbulent flow noise.
The tube may be made as a single element but is more easily formed as separate inlet and outlet steel stampings having mating flanges that are coated with a corrosion preventive and fixed together, such as by rolling or crimping, to form the muffler assembly.
As installed in a PCV system, the orifice muffler reduces the noise level of PCV flow into the intake manifold during engine idle and allows the use of the orifice in place of a PCV valve as is usually required for control of PCV flow directly into an engine intake manifold.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.